Ferrofluid-type seal apparatus are well known and are typically designed to provide a seal about a rotatable shaft to protect the environment on one side of the shaft from contaminants of the environment on the other side of the shaft. Ferrofluid-type exclusion seals are particularly useful with computer-disk-drive spindles to prevent contaminating materials from reaching the disk memory area.
A standard ferrofluid exclusion seal typically comprises an annular, axially polarized permanent magnet adapted to surround a shaft. The magnet is flanked by one or more pole-piece elements which are placed in a contacting magnetic-flux relationship with the polar ends of the permanent magnet. The pole-piece elements are also annular in shape and extend into a close, non-contacting relationship with the surface of the shaft to form radial gaps therebetween. A magnetic fluid or ferrofluid is disposed and magnetically retained in the radial gaps upon insertion of a magnetically permeable shaft, to form one or more liquid O-rings about the shaft which serve to form an exclusion seal.
Many variations to the basic ferrofluid seal have been made to extend the seal life and to reduce the axial length of the seal. U.S. Pat. No. 4,357,021, issued Nov. 2, 1982 to Raj et. al., discloses a seal which utilizes two pole-pieces of unequal width. The ferrofluid under the thinner pole-piece evaporates and forms an air gap while the ferrofluid at the wider, tapered gap remains, providing for extended seal life. Although this configuration exhibits an improved seal life, it is inadequate for applications, such as disk drives, which operate at high ambient temperatures, since the quantity of ferrofluid retained in the gaps between the pole-pieces and the shaft is relatively small.
A prior art seal which has a reduced axial length is disclosed in U.S. Pat. No. 4,407,508, issued Oct. 4, 1983 to Raj et. al. In this patent, a single pole-piece surrounds the shaft to be sealed, and an annular permanent magnet, secured to one surface of the pole-piece. In an alternate embodiment, the single pole-piece has a series of ridges forming a plurality of sealing stages between the pole-piece inner diameter surface and the shaft. Unfortunately, the volume of ferrofluid retained by either embodiment is relatively small. Furthermore, in either embodiment, substantial magnetic flux leakage occurs between the shaft and the magnet, ultimately effecting the seal pressure capacity at the pole-piece.
U.S. Pat. No. 4,506,895, issued Mar. 26, 1985 to Raj, discloses a self-activating, short axial length ferrofluid seal comprising two annular pole-pieces surrounding a thin, large diameter annular magnet. The ferrofluid occupying the gap between the pole-pieces and the permanent magnet forms a sectional, T shaped O-ring seal. Although the seal is self activating and has a short axial length, the life of the seal is again limited by the relatively small amount of ferrofluid in the seal. Furthermore, unless the magnet/pole-piece interfaces are hermetically bonded, fluid leakage is likely to occur at the interfaces.
U.S. Pat. No. 4,628,384, issued Dec. 9, 1986 to Raj et. al., discloses a bearing assembly having an integrated, single pole-piece ferrofluid seal incorporated therein. The annular permanent magnet of the seal has a sectional L-shape which supports the single annular pole-piece. As with prior art exclusion seals, the limited amount of ferrofluid retained by the seal limits the seal life.
U.S. Pat. No. 4,772,032, issued Sept. 20, 1988 to Raj et. al., discloses a compact magnetic fluid low pressure seal in which a bearing assembly is stacked intermediate a single pole-piece and either a radially or axially polarized magnet. As with the other prior art seals, the amount of ferrofluid retained in the seal is relatively small and therefore limits the life of the seal. Furthermore, the above apparatus cannot be used for short axial length applications.
U.S. patent application Ser. No. 07/182,510, filed Apr. 18, 1988, which is copending and co-owned, discloses a two pole tapered magnetic fluid seal which is axially adjacent a magnetically permeable bearing assembly and separated therefrom by a non-magnetic annular spacer. The pole-pieces of the seal axially abut the magnet, with the inner-diameter surfaces of the pole-pieces and magnet tapering linearly to retain the magnetic fluid. Although the seal is capable of retaining an increased quantity of magnetic fluid, the magnet/pole-piece interfaces must be hermetically bonded to prevent magnetic fluid loss.
A further drawback of prior art two-pole magnetic fluid seals is that they cannot be conveniently and reliably filled with both radial gaps activated. In a conventional two pole seal, in order to activate both gaps, the magnetic fluid is added in the magnet region followed by insertion of the shaft into the magnetic assembly. This technique leads to an unequal distribution of magnetic fluid in the radial gaps forming the seals. Furthermore, a quantity of unused fluid is left in the magnet region.
Accordingly, it is therefore an object of the present invention to provide an improved magnetic fluid seal which is capable of retaining a relatively large volume of magnetic fluid thereby extending the seal life.
A further object of the invention is to provide a magnetic fluid seal which reduces fluid leakage between the magnet/pole-piece interface and which eliminates the need to hermetically bond the magnet and pole-piece together.
Another object of the present invention is to provide a long-life magnetic fluid seal which can be utilized in short axial length applications.
A further object of the present invention is to provide a long-life magnetic fluid seal in which the static pressure of the seal changes minimally over an extended time.
Yet another object of the present invention is to provide a magnetic fluid seal which has a low magnetic fluid maniscus despite increased magnetic fluid volume in the seal.
Still a further object of the present invention is to provide a seal assembly which enables convenient and accurate activating of the gaps of multiple pole-pieces with a magnetic fluid.